Investigation of chemical shielding property and its relationship to structure of biomacromolecules using NMR and density functional theory methods

This thesis focuses on the study of chemical shift and structure relationships in biomacromolecules. A principal goal is to develop and apply chemical shift/shielding method for structural study of nucleic acids and for in-depth understanding of the electronic property of metalloporphyrin system.; In the first Chapter, a general review of structural investigation of biological molecules and chemical shift/shielding method is given. Then Chapter Two provides the basic theoretical background in the quantum chemical calculations and solid state NMR fields.; In Chapter Three, chemical shift and structure relationships in nucleic acids are studied and established through the experimental correlations and the understanding of different conformation contributions to the chemical shifts using density functional theory (DFT) method. As the further testing of these chemical shift and structure relationships, the structures of several RNA hairpin loops, a d(GGTCGG) DNA sequence, and a sl26 RNA sequence have been reanalyzed. The results show that the structural information encoded in the observed 13C and 15N chemical shifts may be deduced to provide important torsion angle constraints and base pairing information in nucleic acid structure determination.; Chapter Four concerns with the study of chemical shielding and electronic property of metalloporphyrins. The hybrid DFT method is successfully applied to the calculation of the 59Co chemical shift tensors in hexacoordinated Co(III) porphyrin system. The calculated results of Co(TDCPP)(MeIm) ₂ at B3LYP/6-311G** level using experimental geometry give excellent agreement with the experimental values. Furthermore, the electronic properties of a series of Co(TP_xP)(RIm)₂ (X and R = substituents) has been theoretically demonstrated to give good correlation with the shielding properties of the central metal. It is proposed that electron-releasing substituents on the porphyrinate ligand transfer electron density to the metal not only via the porphyrin e(π) orbitals but also with the participation of porphyrin nitrogens σ orbitals. Moreover, a number of unresolved issues in the literature concerning shielding anisotropy, linewidths, axial ligand substitution, and hydrogen bonding have been addressed and interpreted. It is demonstrated that the hybrid DFT method may provide a way to the study of the electronic and the molecular structure of reasonably large molecules.